Known aircraft comprise a fuselage having a front portion in which a cockpit is arranged, a main portion that bears the wings and a rear fuselage that supports the horizontal and the vertical stabilizers and the auxiliary power system APS.
Usually the rear fuselage comprises a main body and a tail cone end located at the rear part of the main body.
It is known that the interface between main body and tail cone end of the rear fuselage consists of four fittings in both sections and another attachment point called a “balancer” as depicted in FIG. 1.
Due to the load concentration on these interface points, the failure of some of these fittings is one of the critical failure modes specifically for tail cone end sizing which means they have to be robust, thus implying which implies some weight penalties.
Additionally, these fittings, which are usually metallic, require specific maintenance inspections, that are difficult to perform due to lack of space when the tail cone end is small, for instance, for single aisle aircraft.
Therefore, this configuration having two separate sections, main body and tail cone end, leads to higher manufacturing and assembly operations and therefore costs.
The design of the main body of the rear fuselage depends on the high loads introduced by the empennages, mainly the vertical tail plane, VTP, and the horizontal tail plane, HTP, and the design of the tail cone end. The rear fuselage of an aircraft has additionally to be able to withstand loads from the auxiliary power system APS located in the tail cone end.
The structural elements for both elements, main body and tail cone end, are mainly the same as for the rest of the fuselage, that is to say:
Skin that is an integral element of the structure which supports aerodynamic loads. Its function is to provide and maintain the aerodynamic shape, being able to also contribute in its structural resistance.
Longitudinal structural members, for instance, stringers and beams that are primarily responsible for transferring the aerodynamic loads acting on the skin into the frames. Stringers carry loads in the longitudinal direction and provides bending stiffness to the skin reducing the skin thickness, and the beams reinforce cut outs or areas in which local loads are introduced.
Transversal structural members, for instance, frames establish the shape of the fuselage and stiffen the structure transversally, avoid the overall instability of the fuselage and can be subjected to the introduction of local loads.
Both in a T-tail aircraft and in a conventional configuration aircraft, HTP and VTP are attached to the fuselage through the main body and the generated loads are directed to the transversal structural members, more specifically to load frames.
In the main body, the horizontal tail plane, HTP, is considered as interchangeable in some cases, which must be taken into account also in the design.
Additionally, in the design of the rear fuselage other events must be taken into consideration and specifically fire requirements must also be considered.
In the state of the art, it is known to locate the auxiliary power system (APS) in the tail cone as it constitutes a possible source of fire and, as the tail cone is a secondary structure, any possible damage or fault in the tail cone would not imply a catastrophic failure. Therefore, in order to prevent possible fires in the primary structures of the aircraft, the APS is located in the tail cone, furthermore being protected from the rest of the fuselage by means of the fire compartment walls. Given that the APS requires maintenance, the skin of the tail-cone needs to incorporate a door or similar element permitting access to the APS.
In case of aircraft with T-tail configuration, there is also a fire compartment housing the APS, which is also limited in flight direction by a front firewall. This firewall shall be positioned after the high loaded frames or frames withstanding load coming from VTP. In this case, the APS and the fire compartment will be attached to the tail cone end structure as well.
Due to these fire requirements, the fire compartment has front, lateral and rear firewalls, delimited in flight direction by the front firewall which is placed a certain distance behind the main body tail cone end interface, said distance depending on the systems placed at that area and the size of the maintenance door needed for their inspection. Inside the fire compartment the APS is positioned, being the APS and the fire compartment attached to the tail cone end structure, as previously stated.
Considering the HTP interchangeability, state of the art designs have a separate main body and tail cone end in their rear fuselage in order to quickly remove and replace the HTP when the aircraft is on ground.